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PostPosted: June 19, 2017, 1:48 pm 
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Joined: May 27, 2006, 9:46 pm
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Location: BC, Canada. eh?
Too bad you don't have room for a booster, as that would probably cure your problem instantly. I have a really small one that you could have, if you wanted it (I was initially going to use it, rebuilt the master, etc., but in the end I went with floor-mount pedals, so...).

If you like, I can take some measurements for you, just in case it might be possible...

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PostPosted: June 19, 2017, 2:43 pm 
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Joined: August 31, 2015, 2:24 pm
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There are a number of different brake calculator spread sheets on the internet (apparently we can't attach xls sheets to this forum). It's worth getting all the info you have and plugging it into a spreadsheet vs. trying to guess what is going to happen. if want to pm an email I can send you the one I use but it's not setup for drums (got it off 7builder at some point in time). if the pedal is not moving at all make sure you are not binding the balance bar. But really best to work backwards from the wheels with a calculator to get a good starting point otherwise you might get something with a really odd setup.

edit: found it: http://www.locostusa.com/forums/viewtopic.php?p=40140#p40140


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PostPosted: June 19, 2017, 3:14 pm 
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If the pedal is still hard, and not feeling "squishy" such that under hard braking it's using up pedal travel, you can increase braking force more by going to the next size larger on the wheel cylinders. Between that and not using rock-hard pads, you really shouldn't have a need for power brakes.

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PostPosted: June 19, 2017, 10:41 pm 
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Joined: October 24, 2008, 2:13 pm
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Location: Carlsbad, California, USA
hfmaxi wrote:
There are a number of different brake calculator spread sheets on the internet (apparently we can't attach xls sheets to this forum). It's worth getting all the info you have and plugging it into a spreadsheet vs. trying to guess what is going to happen. if want to pm an email I can send you the one I use but it's not setup for drums (got it off 7builder at some point in time). if the pedal is not moving at all make sure you are not binding the balance bar. But really best to work backwards from the wheels with a calculator to get a good starting point otherwise you might get something with a really odd setup.

edit: found it: http://www.locostusa.com/forums/viewtopic.php?p=40140#p40140


That looks like the spreadsheet seattletom updated and improved. Tom sent it to me. It's actually based on Fred Puhn's book, "Brake Handbook." You may need Puhn's book to make sense of it, but once filled in, you can do "what if" scenarios quite easily.

I modified Tom's revamped file to fit the particulars of my build. However, I can see if I have a copy of his original, or you could ask him for it directly. Just let me know if you're interested.

Cheers,

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Build Log: viewtopic.php?f=35&t=5886


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PostPosted: June 20, 2017, 7:51 am 
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Joined: August 31, 2015, 2:24 pm
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Lonnie-S wrote:
That looks like the spreadsheet seattletom updated and improved. Tom sent it to me. It's actually based on Fred Puhn's book, "Brake Handbook." You may need Puhn's book to make sense of it, but once filled in, you can do "what if" scenarios quite easily.

I modified Tom's revamped file to fit the particulars of my build. However, I can see if I have a copy of his original, or you could ask him for it directly. Just let me know if you're interested.

Cheers,


a big thank you to all that helped put it together. I had used it to help get the brakes sorted on my rally car years before starting the roadster project.


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PostPosted: June 21, 2017, 8:01 am 
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Joined: February 28, 2009, 11:09 pm
Posts: 1307
Location: Connersville, Indiana
My experience, which while not directly applicable, should prove helpful.

My car is a Sunbeam Alpine with GM "Metric" front calipers. Piston diameter, 2.5" or 4.9 sq. in. When using a 3/4" (.44 sq.in.) MC, the brakes were good, but the MC would bottom out when braking hard. .44/4.9 gives a cylinder area ratio 11.13:1.

Installed a 13/16" (.52 sq. in.) MC. Pedal is harder, but will not bottom out. .52/4.9 gives a cylinder ratio of 9.4:1

Transferred to your situation:
A 3/4" (.44' sq. in.) MC when combined with 2.01" caliper gives a ratio of .44/3.17 or 7.20:1
A 5/8" (.306 sq. in) MC would give a ratio of 10.35:1, which may or may not bottom out (when used in my setup).
A 1/2" (.2 sq. in.) MC would give a ratio of 15.8:1 and be out of the question.

All of which leads me to think that the 3/4" MC should work fine, perhaps 11/16 would be best, but, as has been suggested, you have a binding issue.

Bill


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PostPosted: June 21, 2017, 8:32 am 
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Joined: September 22, 2005, 8:12 am
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Location: 4AGE in S.E. Michigan
Just to add to Bill's [BBlue] excellent comments. As a general rule for brake master cylinder size. The M/C to caliper areas should be betwwen 7 and 10 ratio when comparing areas. Dave W


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PostPosted: July 19, 2017, 10:02 pm 
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Joined: June 8, 2010, 8:02 pm
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Location: White Rock, BC, Canada
Thanks for the input guys.
I did use a (that?) spreadsheet to get to my starting point. There is no bind in the system, but the more I thought about it, and the more I looked into what other guys were running, the less it made sense to be a hydraulic issue. Just for the hell of it I installed the HP+ brake pads and I now have enough brake grip to very easily lock up the front tires at speed. I also now have a massive front bias, but did not have time to see how bad. I think the pads went bad sitting for so long, and my shoes probably are the same. I also had my right wheel cylinder leak for the second time, and the left was starting. Garbage parts, and maybe don't like the residual pressure valve. There are no other options than Raybestos for these, so the obvious answer is to upgrade to 1988 Tbird disc brakes with HPS pads. I replaced the wheel cylinders for now, and hope to do my first autoX with itthis weekend..... front bias and all.

Cheers.

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